The fuel reformer device that produces the hydrogen rich gas from a hydrocarbon and steam is a known device used to supply a gaseous fuel to fuel cells. The fuel cells convert the chemical energy of a fuel into electrical energy not via mechanical energy or thermal energy but directly, and thus attain a high energy efficiency. In the fuel cells, a gaseous fuel containing hydrogen is supplied to anodes thereof, whereas an oxidizing gas containing oxygen is supplied to cathodes thereof. The fuel cells generate an electromotive force through electrochemical reactions proceeding at both the electrodes. The following equations define the electrochemical reactions proceeding in the fuel cells. Equation (1) represents a reaction proceeding at the anodes, and Equation (2) represents a reaction proceeding at the cathodes. The reaction expressed by Equation (3) accordingly proceeds in the fuel cells as a whole.H2→2H++2e−  (1)(½)O2+2H++2e−→H2O  (2) H2+(½)O2→H2O  (3)
An oxidizing gas and a gaseous fuel containing carbon dioxide are usable in polymer electrolyte fuel cells, phosphate fuel cells, and molten carbonate electrolyte fuel cells among a diversity of fuel cells, because of the properties of their electrolytes. In such fuel cells, the air is generally used for the oxidizing gas, and a hydrogen rich gas produced by steam reforming a hydrocarbon like methanol or natural gas is used for the gaseous fuel. A fuel-cells system using such fuel cells has the fuel reformer device, in which the steam reforming reaction proceeds to produce the gaseous fuel. The following describes the reforming reaction proceeding inside the fuel reformer device. The description regards a case using methanol as the hydrocarbon subjected to the reforming reaction. The following equation represents a reaction of steam reforming methanol:CH3OH+H2O→CO2+3H2−49.5 (kJ/mol)  (4)
As shown by this Equation (4), the steam reforming reaction is endothermic. A supply of thermal energy is accordingly required for the progress of the reforming reaction. A known method for supplying thermal energy required for the reforming reaction externally applies heat by means of a burner or a heater installed in the fuel reformer device. Another known method causes an exothermic oxidation reaction to proceed in addition to the steam reforming reaction in the fuel reformer device and utilizes the heat generated by the oxidation reaction for the progress of the steam reforming reaction. Among these known methods, the method of causing the oxidation reaction to proceed in parallel with the steam reforming reaction in the fuel reformer device is discussed below.CH3OH+({fraction (1/2)})O2→CO2+2H2+189.5 (kJ/mol)  (5)
Equation (5) represents an example of the oxidation reaction of methanol (partial oxidation reaction). In the structure that introduces a supply of oxygen into the fuel reformer device and causes the oxidation reaction of methanol to proceed in parallel with the steam reforming reaction expressed by Equation (4), the thermal energy generated by the oxidation reaction is utilized for the steam reforming reaction. Regulating the flow rate of oxygen supplied to the fuel reformer device enables the amount of heat required for the steam reforming reaction to balance the amount of heat generated by the oxidation reaction. Theoretically the quantity of heat generated by the oxidation reaction may compensate for the quantity of heat required for the steam reforming reaction. Compared with the external heating method, this method of making the amount of heat generated by the oxidation reaction supply the amount of heat required for the steam reforming reaction has the less energy loss due to heat dissipation and attains the higher energy efficiency. Compared with the external heating method, this method simplifies the structure of the fuel reformer device and enables the size reduction of the whole fuel cells system.
The method of supplying oxygen as well as methanol and steam to the fuel reformer device and utilizing the thermal energy generated by the oxidation reaction for the steam reforming reaction, however, has a drawback, that is, an uneven temperature distribution inside the fuel reformer device. FIG. 38 is a graph showing a temperature distribution inside the fuel reformer device that receives supplies of oxygen as well as methanol and steam and causes the oxidation reaction to proceed in parallel with the steam reforming reaction. When a supply of oxygen is introduced together with supplies of methanol and steam into the fuel reformer device, since the oxidation reaction has the higher reaction rate than the steam reforming reaction, the amount of heat generated by the oxidation reaction exceeds the amount of heat required for the steam reforming reaction on the upper stream side in the fuel reformer unit (that is, the side receiving a supply of the gas containing methanol, steam, and oxygen). As shown in the graph of FIG. 38, the internal temperature abruptly rises on the upper stream side to form a peak in the temperature distribution. After the consumption of oxygen by the oxidation reaction, only the steam reforming reaction proceeds. The internal temperature of the fuel reformer device thus continuously decreases towards the lower stream side (that is, the side outputting the hydrogen rich gas) after the peak in the temperature distribution.
Formation of the peak in the temperature distribution or the excessive temperature rise inside the fuel reformer device results in some drawbacks, for example, deterioration of the catalyst and formation of by-products. One problem is deterioration of the catalyst. It is here assumed that a Cu—Zn catalyst is used for the catalyst of accelerating the steam reforming reaction and the oxidation reaction of methanol. The use of the Cu—Zn catalyst at high temperatures over 300° C. lowers the durability of the catalyst and may cause sintering. Sintering is the phenomenon that the catalyst carried on the surface of the carrier aggregates. The Cu—Zn catalyst is generally formed by dispersing copper fine particles on the surface of zinc particles. The occurrence of sintering causes the copper fine particles to aggregate and form giant particles. This phenomenon decreases the surface area of the copper particles and reduces the area of the catalytic activity, thereby lowering the performance of the fuel reformer device.
Another problem due to the excessive rise of the catalytic temperature is formation of by-products. A side reaction other than the normal reforming reaction discussed above occurs in parallel with the reforming reaction at preset high temperatures to form methane. Gaseous nitrogen included in the supply of the pressurized gas undergoes a side reaction to produce nitrogen oxides. These by-produces are not decomposed in the temperature range of the reforming reaction in the fuel reformer device, but are supplied to the fuel cells as part of the gaseous fuel. An increase in quantity of the by-products like methane unfavorably lowers the hydrogen partial pressure of the gaseous fuel.
The decrease in internal temperature on the lower stream side of the fuel reformer device disadvantageously lowers the activity of the steam reforming reaction. The lowered activity of the steam reforming reaction may cause the non-reformed gas, that is, methanol, to remain and give a resulting gas having an insufficiently low concentration of hydrogen. In order to ensure the completion of the reforming reaction even under the condition of the low internal temperature on the lower stream side, a sufficiently large fuel reformer device is required.
The object of the present invention is thus to solve such drawbacks and to keep the internal temperature of the fuel reformer device within a preset temperature range.